1. Technical Field
The present invention relates to a light source apparatus and a projector.
2. Related Art
Using a solid-state light source as a light source of a liquid crystal projector on a commercial basis is underway (see JP-A-2012-47996). Blue light from a laser or any other solid-state light source is caused to impinge on a phosphor adhering to a rotating disk so as to cause the phosphor to emit yellow light. The yellow light can be combined with blue light from another laser to form white light. Light valves formed of three liquid crystal panels produce video images of three colors, which are combined with one another by a prism, and the combined video images are projected through a projection lens on a screen. A solid-state light source is characterized by a long period during which brightness is maintained, significantly low probability of blowout, and environmental friendliness owing to no mercury content as compared with a mercury lamp of related art and hence considered as a technology that further advances in the future. YAG containing Ce ions is frequently used as the phosphor and has excellent reliability because it is hardly degraded at a high temperature of 200° C.
The phosphor converts the wavelength of excitation light to produce light having a wavelength longer than the wavelength of the excitation light. Quantum dots also have the wavelength conversion function (see JP-A-2013-229145).
A YAG phosphor containing Ce ions is diffused into a binder made, for example, of a silicone resin or a glass material, and the mixture of the phosphor and the binder is fixed in the form of a thin film onto a rotating disk, or a YAG phosphor ceramic in the form of a thin film itself is fixed onto a rotating disk without using a binder. The thus fixed phosphor is irradiated with blue laser light, which excites the Ce ions and causes them to emit yellow light. The phosphor is thus used as a light source for a projector. In this process, at the time of the wavelength conversion, in which the blue light is converted into the yellow light, energy loss (Stokes loss) occurs, resulting in heat generation. Since the light source of a projector is required to be a point light source that emits intense light, a wide excitation light beam is applied onto a phosphor in the form of a small spot. An excitation section is therefore partially heated to a high temperature. To dissipate the heat into the air, the rotating disk is rotated at high speed. A silicone resin and a phosphor have low thermal conductivity ranging from about 0.2 to 12 W/mK and are also characterized in that the thermal conductivity thereof decreases as the temperature increases. The heat dissipation is therefore insufficient, and the silicone resin is degraded due to high temperatures, resulting in a decrease in the conversion efficiency of the YAG phosphor itself. Further, a part of the light source is broken depending on input energy. To understand the energy inputted to a phosphor light source, the phosphor light source is compared with a point light source formed of a 200-W ultrahigh-pressure mercury lamp having an arc length of 1 mm having been used in related art by way of example. An ultrahigh-pressure mercury lamp has a light emission efficiency of about 25%, and the amount of emitted light is therefore 50 W. To replace the ultrahigh-pressure mercury lamp with a YAG phosphor, an excitation section having a size of 1 mm needs to emit intense light of 50 W. Assuming that the phosphor has a conversion efficiency of 50%, heat equivalent to 50 W is generated. Heat equivalent to about 50 W is therefore generated in a spot of 1 mm square. When the rotating disk has a diameter of several centimeters, the silicone resin is heated to a temperature higher than 200° C., which degrades the silicon resin. It is, of course, conceivable to increase the number of revolutions of the rotating disk and the radius thereof, but countermeasures against the heat are still required in consideration of the facts that the amount of heat transfer from the rotating disk to the air is small and rotation power of a motor is limited.